Field of the Invention
This invention relates generally to deposition systems and methods, and in particular to systems and methods for use in connection with atomic layer deposition processes, such as growing a thin film onto a substrate through plasma-enhanced atomic layer deposition (PEALD).
Description of the Related Art
As is known in the art of deposition technologies, atomic layer deposition (ALD) is a chemical vapor deposition (CVD) technique that enables surface-controlled film growth on the atomic scale with excellent uniformity. Surface-controlled film growth is a unique feature that is based on sequential, self-limiting chemical reactions between gas-phase precursor molecules and active surface species. During a typical ALD process, at least two gas-phase precursors are pulsed sequentially into a reaction space where a substrate is located. A complete sequence (or cycle) is made up of a series of pulse and purge steps. Pulse steps are separated by purge steps to remove any remaining precursor and/or volatile reaction byproducts from reaction space between pulses. Thus, a complete cycle requires a minimum of four steps.
During each pulse step, chemical reactions between precursor molecules and active surface species yield new surface species that passivate the surface. Once the surface becomes fully passivated, reactions are complete and result in the formation of a limited number of new surface species. Uniformity depends primarily on the distribution of active surface species/sites and completion of surface reactions during each precursor pulse step. Subsequently, remaining precursor and/or reaction byproducts are purged in preparation for next pulse step. A complete cycle is required to obtain the desired material. Each cycle deposits a very specific amount of material onto the substrate surface and is repeated until the desired amount of material has been deposited, enabling very accurate control of film thickness. Typical growth per cycle (GPC) is approximately 1 Å/cycle, but depends on the specific process.
Known thermal-based methods depend on substrate heating where the range of process temperatures resulting in ideal growth is referred to as the ALD window. Ideal growth is characterized by chemical adsorption of surface species that is irreversible, self-limiting, and complete. Outside of this temperature window, growth becomes non-ideal. At temperatures below the ALD window, thermal energy at the substrate surface becomes insufficient for surface reactions and/or to prevent physical adsorption (or condensation) of precursor molecules. Thermal self-decomposition of precursor molecules and/or desorption of chemically-adsorbed surface species result in non-ideal growth at higher substrate temperatures. In many cases, the ALD window is broad enough to enable ALD of different materials (e.g., multilayer film growth) at a constant substrate temperature.
PEALD methods utilize reactive plasma species as precursors for ALD surface reactions. Typical plasma gases include O2, N2 and H2. Plasma technology is well established in other areas of thin film processing, such as physical vapor deposition (PVD), conventional CVD, and reactive ion etch (RIE) methods, and has been applied to ALD techniques as well. Benefits of PEALD include lower temperature process capability, as well as new pathways for chemical reactions that would otherwise be inaccessible by purely thermal methods. Plasma treatments can also be used for substrate surface modification prior to ALD processing.
Advantages of ALD methods include uniform, conformal surface coverage with atomic scale thickness and composition control. Sequential precursor pulsing eliminates the potential for gas-phase reactions that result in film defects so that highly reactive precursors can be utilized. Highly reactive precursors yield dense, continuous films with low levels of residual contamination and defects at relatively low process temperatures. High quality materials demonstrated include oxides, nitrides, metals, as well as complex multi-component films and multi-layered structures. Since ALD is primarily determined by precursors, substrate temperature, and starting surface, process is highly reproducible and scalable, thereby simplifying scale-up from research and development to production. These advantages are directly related to the unique surface-controlled growth mechanism of ALD.
One known limitation of existing ALD processing is the relatively low throughput due to long process times when compared to PVD and conventional CVD methods. However, this limitation has become less significant in recent years due to reduced scaling of microelectronic device dimensions and interest in other areas of nanotechnology, where advantages of the surface-controlled growth mechanism of ALD are paramount. Another disadvantage is the limited number of precursors presently available and, therefore, materials deposited by ALD methods. Precursor and process development are very active areas of research and development (R&D) activity.
Film uniformity, as described above, depends primarily on distribution of active surface sites and completion of surface reactions during each precursor pulse step. Sources of ALD non-uniformity have been reviewed in an article by K.-E. Elers et al., Chem. Vap. Deposition 12, 13 (2006), which is incorporated herein by reference. These sources of non-uniformity include overlapping of precursor pulses, non-uniform precursor delivery, and precursor thermal self-decomposition. In addition, reaction byproducts have been shown to contribute to film non-uniformity. The most prominent of these, however, is overlapping of precursor pulses. Adequate purging between precursor pulse steps is critical, since overlapping of pulses results in parasitic CVD surface reactions that decrease film uniformity and increase the potential for gas phase reactions and associated film defects. Moreover, rapid purging is necessary to reduce overall process times since cycle times are largely determined by the duration of purge steps.
Deposition is generally accomplished by placing the substrate to be coated into a vacuum chamber where it is heated to process temperature. Chamber volume is in communication with ports for vacuum and various inputs for introducing precursors and inactive gases. ALD apparatus designs are classified as perpendicular or parallel flow-type, based on net direction of gas flow relative to the substrate surface. Perpendicular flow designs are characterized by net gas flow that is perpendicular, and parallel (or cross-flow) designs by gas flow that is parallel, to the substrate surface. Precursor pulse valves should be located in close proximity to the chamber to enable efficient purging. In addition, chamber volume should be minimized and inputs, chamber, and foreline virtually free of pockets (or dead-space volume). Adequate pumping, as well as heating of all input, chamber, and foreline components is essential for efficient purging between precursor pulse steps.
Typically, inactive gas flow is continuous and used to help efficiently remove remaining precursor and/or reaction byproducts between pulse steps. During pulse steps, inactive gas flow serves as a carrier gas to help transport precursors to the substrate surface. Efficient precursor delivery is beneficial to complete surface reactions quickly and without significant waste.
Limiting exposure of chamber surfaces between point of precursor injection and the substrate surface minimizes pulse times and precursor consumption. Furthermore, ALD surface chemistry is not always ideal, such that film uniformity depends on distribution of precursor across the substrate surface during pulse steps. For example, precursor thermal self-decomposition produces film thickness gradients determined by the precursor distribution across the substrate surface. Plasma processes may also require a reasonably uniform distribution of reactive plasma species across the substrate surface to achieve good film uniformity. Perpendicular flow-type designs are well suited to address these sources of ALD film non-uniformity.
With reference to the PEALD process, plasmas are generated by coupling an applied electromagnetic field to the process gas. Energy is transmitted to the process gas through acceleration of electrons by the applied field. Subsequent gas decomposition yields electrons, ions, and radicals (i.e., plasma). Inductive, radio frequency (RF) plasma sources are quite common (e.g., 13.56 MHz frequency) and routinely utilized to generate plasma for thin film processes.
Inductively coupled plasma (ICP) sources generate high-density plasma and are considered remote when the substrate is not directly exposed to the plasma generation region. The remote plasma configuration minimizes potential for substrate surface damage due to direct plasma exposure. Source electrode (or antenna) is typically mounted ex-situ, such that RF radiation is transmitted through a dielectric window (e.g., quartz or alumina) with relatively low sputter yield. Therefore, only the surface of the dielectric window is exposed directly to plasma, thereby eliminating the potential for ALD film contamination due to sputtering of the metallic electrode material. In addition, by operating in an inductive mode, the potential for film contamination due to sputtering of the dielectric window surface is minimized.
Plasma source integration, in general, increases process chamber design complexity and volume. Remote ICP sources are generally detachable and connected directly to the main vacuum chamber where the substrate is located. Hence, source is easily removable for servicing and/or when purely-thermal ALD configuration is desired. To obtain a uniform plasma distribution, the plasma port should be positioned above, and vacuum port below, the substrate surface. Ideally, the plasma and vacuum ports share a common central axis that is perpendicular to, and passes through the center of, the substrate surface. During precursor pulsing, it is important to avoid condensation of precursors (e.g., water) on the internal surface of the plasma source dielectric window that can become a source of parasitic CVD during subsequent precursor pulse steps. It is also very important to prevent deposition of absorbing layers on this surface, since this will result in attenuation of the RF signal and subsequent plasma source failure. Isolation of the plasma generation region from precursors (other than plasma process gases) may be achieved by incorporating an isolation valve; however, the cycling of an isolation valve during ALD can produce particles resulting in unwanted film defects.
Other apparatus design features include real-time process monitoring and cluster tool integration capabilities. Like plasma, the integration of these features increases chamber design complexity and volume. Spectroscopic ellipsometry (SE) is a powerful and effective method for real-time process control and monitoring. SE is a non-destructive thin film characterization technique that utilizes specularly-reflected polarized light to determine properties such as film thickness and optical constants. Integration typically requires at least two view-ports for mounting spectrometer components and transmission of light into, and out of, the ALD process chamber. Problems associated with the integration of chamber ports necessary for SE include coating of the transparent windows resulting in attenuation of transmitted light, as well as more complex chamber geometry and additional volume that increases required purge times.
Cluster tool integration enables multi-technique capabilities without having to break system vacuum and exposing potentially sensitive substrate surfaces to atmosphere prior to subsequent processing. These multi-technique capabilities include other thin film process modules, as well as in-situ analytical techniques (e.g., x-ray and Auger photoelectron spectroscopy, four-point probe, etc.) and single/cassette wafer loading modules. To enable vacuum transfer an additional chamber port is necessary. Similar to ports for SE integration, this additional port for substrate transfer increases chamber design complexity and volume and, therefore, required purge times.
It is noted that certain existing ALD systems (primarily for R&D activities) have more limited capability and do not include extra ports or the associated volume necessary for the enhanced design features described above. In some cases, these relatively simple systems enable thermal processes only and have no diagnostic or vacuum transfer capability. Therefore, internal volume is relatively small and all surfaces within the main process chamber (or apparatus) are equally exposed to the ALD precursors. As ports are added to accommodate enhanced process capabilities, inactive curtain gas may be employed to help keep precursors out of port volumes and to remove any precursors and/or reaction byproduct that may enter these ports during pulse steps. The combination of remote plasma, as well as chamber ports necessary for SE and vacuum transfer capability (primarily for more advanced R&D activities) requires specialized processing requirements and design consideration, as discussed above. For example, the process chamber volume increases significantly for at least two reasons. First, for plasma to be “remote” there must necessarily be sufficient spatial separation between the plasma generation region and substrate surface. In addition, spatial confinement of the plasma results in quenching of active species necessary for ALD surface reactions. Thus, remote plasma source integration requires a significant increase in internal volume to satisfy plasma related performance requirements.
Still further, extra chamber ports with associated volume are necessary to enable SE as well as vacuum transfer capability. The angle of incidence for SE is generally around 70° from the axis perpendicular to the substrate surface such that spatial requirements for SE and remote plasma integration are somewhat complimentary (i.e., volume required for one is advantageous for the other) making the combination desirable for advanced R&D applications (e.g., applications that take advantage of dual thermal and remote plasma process capability including in-situ, real-time SE and cluster tool integration). These enhanced system features, however, significantly increase process chamber design complexity to maintain ALD process performance.
In certain environments, curtain gas will be used to help protect plasma, analytical, and transfer port volumes. In addition, mechanical shutters and/or valves are usually necessary for one or more of these ports to help eliminate flow and/or diffusion of precursors into port volumes. For example, gate valves can be used to isolate the analytical view ports during precursor pulse steps. If these gate valves are not closed during precursor pulse steps, then the transparent windows for SE light transmission can become coated, resulting in limited ability to monitor process due to signal attenuation. Similar signal attenuation issues can occur when the remote plasma source is not properly protected during precursor pulse steps. This is due to deposition of absorbing materials on the transparent dielectric window resulting in plasma source failure. In either case, system venting and maintenance are required to rectify these issues.
According to certain existing systems, the cycling of mechanical valves during processing can generate particles that result in unwanted film defects. In particular, remote plasma source isolation can be most problematic since a gate valve is typically positioned directly above the substrate surface where particles tend to rain down onto the surface when the valve is cycled. In cases where an inactive gas barrier is used to prevent film deposition inside the plasma source (without the use of a mechanical gate valve), it is still difficult to prevent diffusion of certain precursors into the plasma port during precursor pulse steps. For example, water is the most common oxidant used for thermal ALD process. During precursor pulse steps, water diffusion and subsequent condensation inside the remote plasma port can become a source of parasitic CVD that severely limits thermal ALD process performance.
In summary, there are many challenges associated with the integration of various ALD apparatus design features. For example, such features include remote plasma, SE for real-time process monitoring, and cluster tool integration capabilities for advanced, single wafer R&D applications. Due to more complex process chamber geometry, as well as additional volume requirements, these enhanced system features significantly increase apparatus design complexity to maintain ALD process performance (e.g., uniformity and cycle times). Additional challenges associated with remote plasma integration include plasma performance issues associated with exposure of internal plasma source components to ALD precursors (other than plasma process gases). Similar challenges are associated with the integration of analytical ports necessary for SE, where exposure of internal components to ALD precursors can result in issues with feature performance. Accordingly, there is a need in the art for an atomic layer deposition apparatus and process that recognizes and addresses some or all of these challenges and issues.